1. Field
Embodiments of the present invention relate to a radiation source, and to a lithographic apparatus which is in connection with or includes such a radiation source.
2. Background
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to be able to project ever smaller structures onto substrates, it has been proposed to use extreme ultraviolet (EUV) radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm. It has further been proposed that radiation with a wavelength of less than 10 nm could be used, for example 6.7 nm or 6.8 nm. In the context of lithography, wavelengths of less than 10 nm are sometimes referred to as ‘beyond EUV’.
Extreme ultraviolet radiation and beyond EUV radiation may be produced using a plasma. The plasma may be created for example by directing a laser at particles of a suitable material (e.g., tin), or by directing a laser at a stream of a suitable gas or vapor, such as Xe gas or Li vapor. Alternatively, the plasma may be created using an electrical discharge. The resulting plasma emits extreme ultraviolet radiation (or beyond EUV radiation), which is collected using a collector such as a normal incidence collector or mirrored grazing incidence collector, which receives the extreme ultraviolet radiation and focuses the radiation into a beam.
In addition to extreme ultraviolet radiation, the plasma produces debris in the form of particles, such as thermalized atoms, ions, nanoclusters, and/or microparticles. The debris is projected, together with the extreme ultraviolet radiation, towards the collector and may cause damage to the collector.
It is desirable to prevent debris from coming into contact with and, for example, coating or damaging the collector. Coating of the collector may, for example, reduce the reflectivity of the collector, reducing the amount of radiation that may be collected and used in the patterning of a substrate.
Debris from plasma-based extreme ultraviolet radiation sources is commonly suppressed using a buffer gas. Debris repeatedly collides with constituent parts (e.g., atoms or molecules) of the buffer gas, and these collisions cause the debris to slow down and/or be deflected from their original path. The slowing down and/or deflection of the debris can be used to obviate or mitigate the problem of the debris coming into contact with the collector. After the debris has been slowed down and/or deflected, the debris may, for example, be pumped away (e.g., out of the radiation source) and/or intercepted by a debris trap (for example, a foil trap or the like).
The degree to which the debris is suppressed (i.e., the suppression factor) depends on the number of buffer gas atoms (or, for example, molecules) that debris (for example, a debris atom or the like) encounters on its way through the buffer gas. At constant temperature and volume, the number of buffer gas atoms is proportional to the buffer gas pressure (from the ideal gas law pV=nRT). The buffer gas is often characterized in terms of the integrated pressure along the trajectory of the debris. The suppression can be improved by increasing the integrated pressure. Increasing the integrated pressure can be achieved by increasing the pressure, or by increasing the distance over which the pressure is applied. However, both of these solutions are difficult to implement in practice. For instance, the pressure is typically limited by a maximum operating pressure of the radiation source, since too high a pressure inhibits the expansion of the plasma that emits extreme ultraviolet radiation. The distance over which the pressure may be applied is limited by the space between the point at which radiation is generated (i.e., the location of the radiation emitter, for example, the plasma) and the collector. Increasing this distance increases the size of the radiation source, which is undesirable.